Atrial fibrillation is a common disorder of the heart rhythm, and the prevalence of atrial fibrillation increases with age. In epidemiologic studies, the prevalence of atrial fibrillation is 1% for subjects age 50-59 years, but it increases to 9% at ages 80-89 years. The presence of atrial fibrillation markedly increases the risk of stroke. Fortunately, anticoagulation with warfarin is effective treatment for the stroke risk. In addition to the increased risk of stroke, patients with atrial fibrillation are often symptomatic with palpitations, dyspnea, fatigue, and dizziness; and symptoms are disabling in some patients. A variety of strategies have been developed to treat patients with atrial fibrillation. The use of Class I or Class III antiarrhythmic drugs to restore sinus rhythm and to reduce the frequency of recurrences of atrial fibrillation are the primary therapy. While this strategy has been shown to effectively reduce the frequency of symptomatic recurrences of atrial fibrillation, concerns about the safety of antiarrhythmic drugs have increased interest in alternative strategies including a simpler strategy of controlling the heart rate during atrial fibrillation (with beta blockers, digoxin, verapamil or diltiazem). Some patients are being treated with His bundle ablation and implantation of a permanent pacemaker, which eliminates the need for all drug therapy except for anticoagulation. At the present time, the effect on survival of these alternative strategies for management of patients with atrial fibrillation is simply unknown. The overall goal of the AFFIRM trial is to determine whether a strategy of maintaining sinus rhythm with class I or class III antiarrhythmic drugs is different from a strategy of simply controlling rate with respect to mortality. In the AFFIRM trial, patients are randomly assigned to one of these two competing strategies. Once that assignment has been made, investigators have wide latitude in deciding exactly which drugs or devices to use. The protocol will enroll 5,000 patients. The primary outcome variable is mortality. Almost half the patients have already been recruited.